Lethe was a former Colonised World and Chapter Homeworld of the now-extinct Flame Falcons Space Marine Chapter. Located on the border of the Manachean Commonwealth, the aggregate of advanced and populated worlds, set at the junction point where the Segmentum Obscurus ends and the northeastern sweep of the Ultima Segmentum presses onwards to its outermost limits. At the dawning of the Horus Heresy, this region of space was without doubt the most strategic vital area of the Coronid Deeps. Clustered with inhabitable stars, many of which had possessed human populations since before the Age of Strife, the Commonwealth served as a lynchpin of commerce and trade in this region of the burgeoning Imperium. Vitally, it was also a powerhouse of industry and military endeavour which held the shape of the Imperium's borders by anchoring much of the ongoing force projection of the Imperialis Armada in the wider region.
The world of Lethe was home to a human remnant civilisation which had decayed to a borderline pre-industrial state during the long night of the Age of Strife, Lethe was first contacted in 824.M30 by the Rogue Trader Vaspperly Elgin, ahead of the Great Crusade's advance in force into the region some years later. Used as an early staging point by Crusade fleets, the mist-shrouded, largely arboreal world, criss-crossed by an extensive network of rivers around which its population was clustered, remained still stubbornly a frontier world, without major industrial or economic power having developed there. Nevertheless, Lethe had served its part within the Imperial fold, and had also provided several elite pioneer light infantry cohorts for the Imperialis Auxilia, which served with honoured distinction in the wars to further pacify the Coronid Deeps.
Many millennia later, Lethe would once again find itself written into the annals of Imperial history. Shortly before the start of the Age of Apostasy, sometime in the 36th Millennium, the largest Founding of Space Marine Chapters since the Second Founding took place. This was to be known as the 21st Founding, which would later be referred to as the "Cursed" Founding, because of the appalling bad luck that seems to dog the Chapters that were created by it. One of these Chapters created during this Founding, were the ill-fated Flame Falcons Chapter. Next to nothing is known of the Flame Falcons Chapter in official Imperial records, for they were declared Excommunicate Traitoris within a standard century of their inception. Initial combat trials involving the Chapter were a resounding success, and they were put on active service on the Southern Rim of the galaxy, based on their homeworld of Lethe.
Their first engagements, with Orks and Eldar Corsairs, promised a great future for the Chapter, as they were observed to be hardy and courageous, even for Space Marines. It was at the battle of Raffenburg's World that things began to go awry for the Flame Falcons. At the height of the battle, their largest engagement to that point, the 1st Company were observed to spontaneously immolate at the height of the fiercest fighting against the rebel forces. Fearing witchcraft, the Chapter's commander reinforced the front line and sent for an Inquisitor who was accompanying the Imperial Guard forces. When it was reported by the reinforcing Space Marines that the members of the 1st Company were not dead, but were in fact fighting with renewed savagery covered in flames to which they themselves were immune, the commander declared it a miracle, a gift sent from the Emperor.
The Inquisitor had a different opinion. Knowing that he could not assault the Flame Falcons immediately, he bided his time, witnessing the remainder of the Chapter undergo the same horrific change as the 1st Company. Upon the successful completion of the campaign, the Flame Falcons returned to Lethe in a celebratory mood, convinced, as they were, that their unique ability was a manifestation of the Emperor's grace. Soon after, the Grey Knights, the Chamber Militant of the daemon-hunting Ordo Malleus, at the behest of the Inquisition, assaulted Lethe, ruthlessly destroying the supposedly "cursed" Chapter. To ensure that their taint didn't affect the rest of the planet, Lethe was subjected to Exterminatus. It is thought that a small number of Flame Falcons escaped that day, but of their fate, nothing is known.
- Chapter Approved: The Fourth Book of the Astronomican (2004), "The Cursed Founding" by Andy Hoare, Graham McNeill & Phil Kelly, pp. 4-9
- Codex: Ultramarines (2nd Edition), pg. 13
- The Horus Heresy - Book Four: Conquest, pp. 37, 44, 69, 73, 158, 160
- White Dwarf 279 (UK), "Chapter Approved: The Cursed Founding" by Andy Chambers, Andy Hoare, Graham McNeill & Phil Kelly, pp. 51-52
- White Dwarf 260 (UK), "Index Astartes: The Cursed Founding"